Fluorinated ion exchange polymers having carboxylic acid and/or sulfonic acid functional groups or salts thereof are known in the art. One principal use of such polymers is as a component of a membrane used to separate the anode and cathode compartments of a chloralkali electrolysis cell. Such membrane can be in the form of a reinforced or unreinforced film or laminar structure.
It is desirable for use in a chloralkali cell that a membrane provides for operation at low voltage and high current efficiency, and thereby at low power consumption, so as to provide products of high purity at low cost, especially in view of today's steadily increasing cost of energy. It is also desirable that the membrane be tough, so as to resist damage during fabrication and installation in such a cell. As films of the best available ion exchange polymers have low tear strength, it has been found necessary to strengthen them by fabricating membranes with reinforcement therein, such as a reinforcing fabric.
However, use of reinforcement within the membrane is not totally beneficial. One deleterious effect is that use of reinforcement such as fabric results in a thicker membrane, which in turn leads to operation at higher voltage because the greater thickness has a higher electrical resistance. Efforts to lower the resistance by using thinner films in fabricating reinforced membranes are often unsuccessful because the film ruptures in some of the windows of the fabric during membrane fabrication, resulting in a membrane with leaks. (By "windows" is meant the open areas of a fabric between adjacent threads of the fabric.) A membrane which leaks is undesirable as it permits anolyte and catholyte to flow into the opposite cell compartment, thereby lowering the current efficiency and contaminating the products made. Additionally, thick layers of polymer at the junctions of threads in a reinforcing fabric also constitute regions of high resistance. (By "junctions" is meant the crossover points where threads in the warp meet threads in the weft.)
A second deleterious effect, which also leads to operation at higher voltage, is caused by a "shadowing" effect of the reinforcing members. The shortest path for an ion through a membrane is a straight perpendicular path from one surface to the other surface. Reinforcement members are uniformly fabricated of substance which is not ion-permeable. Those parts of a membrane where an ion cannot travel perpendicularly straight through the membrane, and from which the ion must take a circuitous path around a reinforcng member, are termed "shadowed areas". Introduction of shadowed areas into a membrane by use of reinforcement in effect leads to a reduction in the portion of the membrane which actively transports ions, and thus increases the operating voltage of the membrane. That part of the shadowed area of a membrane which is adjacent the downstream side of the reinforcement members, "downstream" referring to the direction of the positive ion flux through the membrane, is termed the "blind area".
It is a principal object of this invention to provide an ion exchange membrane which operates at low voltage and high current efficiency, and thereby at low power consumption, and yet has good tear resistance. Another object is to provide a thin, tough ion exchange membrane. Other objects will be apparent hereinbelow.